Communication system



July 16, 1940. H. J. M CREARY COMMUNICATION SYSTEM Filed 001;. 19, 1939 2 Sheets-Sheet 1 INVENTOR. HAROLDJ MCREARY W5 ATTORNEY:

July 16, 1940. H. J. M CREARY COMMUNICATION SYSTEM 2 Sheets-Sheet 2 Filed 001;. 19, 1939 INVENTOR. HAROLD J.MCR EARY I Ff FIG.4

Patented July 16, 1940 UNITED STATES PATENT OFF ICE The invention relates generally to communication systems, and more particularly to such systems of the two-way, loud-speaking type wherein each station comprises both a microphone and a loud speaker, and wherein speech impressed on the microphone of either station is reproduced at a comparatively high level or intensity by the loud Y speaker of the other of the station.

With conventional systems of this general character, it is well known that there is often an objectionable feed-back of the high level reproduced sound through the microphone of the "receiving station. Such objectionable feedback takes place principally at the higher levels.

of sound intensity, and the magnitude of 1 the feed-back may vary considerably with many factors of installation and design. However, as a general ibservatlon, it may be said that the feedback for any particular installation becomes dis- 2() tinctly objectionable when the amplification factor exceeds a particular value which may for present purposes be termed the "howl level or "howl threshold.

Because of this objectionable characteristic in systems of this character, it isoften found necessary to abandon the desired automatic or constant connection characteristics of the systems, and to interpose a two-way manual control switch whereby the user may connect the channels for either speaking or listening. Such systems with their manual control key may be stantly connected two-way. loud speaking com-.

"ihunicationsystem wherein a vocal or other sound wave impressed upon and reproduced by either one of the communication channels is .ineffcctive to produce objectionable feed-back 50 of vsuch sound along the other of the channels,

whereby to prevent the objectionable sound int ..:rerence and noise above discussed.

A further object of the present invention is to provide such an automatic system wherein the 55 sound transmitted and reproduced through the agency of either of the two communication channels provides and serves as a controlling force acting to suppress objectionable feed-back of such reproduced sound in the other of the channels.

A further object of the invention is to provide 5 such an automatic or constantly connected twoway loud-speaking communication system wherein a'relatively low normal gain in both communication channels is automatically varied by 10 sound-induced currents in either one of the channels to decrease .the normal gain in the s other or return channel and to increase the normal gain in the first or primary channel; audit is a, further and related object of the invention to 15 produce such a system wherein sensitivity and responsiveness as to such voice-induced variation is facilitated through the use of a relatively high voltage voice-induced control current in combination with automatic means operable to go limit the voice-induced variation in gain in either or both of the channels.

A further object is to provide such a two-way constantly connected communication system wherein the microphone means and the reproa ducing means at each station are so arranged and related in an acoustic sense that the sounds of the predominating frequency of the system reproduced after transmission through one of the channels are, in effect, cancelled out in the re- 39 turn channel, and are thereby rendered ineflective to actuate the loud speaker at the original source of the sound, this objective being attained by means which is ineflective to cancel out the predominating frequency in the primary channel. 35

A further object is to provide such a two-way system wherein acoustic means for reducing the effective feed-back is combined with automatic voice-induced reduction of the amplification factor in the return channel.

Other objects and advantages will become apparent from the following description, taken in connection with the accompanying drawings, in which:

1 Fig. 1 is a diagrammatic illustration of a communication system embodying the invention.

Figs. 2, 2a and 2b are diagrammatic illustrations of alternative forms of the invention wherein feed-back is reduced by acoustic treatment.

Fig. 3 is a diagrammatic illustration of a secand alternative embodiment of the invention wherein the desired result is attained through a combination of acoustic and electrical means.

i Fig. i is a wiring diagram showing the circui employed in the embodiment of Fig. 1. 1

In the form chosen for disclosure in Figs. 1 and 4 of the drawings, the invention is embodied in an automatic two-way loud-speaking communication system having two similar terminal sta tions or sets [a and lllb which in practice are located remote from each other, as in separate rooms II and I2. Each terminal set comprises a microphone i4 and a loudspeaker i5, these elements of the two units or sets being distinguished by the use of the letters a and b in accordance with the set'to which they belong. Thus the microphone Ma and the loudspeaker I 5a form the elements of the set Illa, and may be mounted in a suitable cabinet [6a, indicated in dotted outline in Fig. 4, while the corresponding elements of the set Hlb may be mounted in a similar cabinet SD.

The two sets 10a and I0?) are interconnected through amplifying and control means which in Fig. 1 is indicated generally by the reference numeral 20, there being a-first transmission path or channel 2| between the microphone Ma and the loud speaker l5b, and a second transmission path or channel 22 between the microphone it?) and the loud speaker I5a.

In a conventional automatic system of the general character above described, where both transmission paths, as the paths 2! and 22, are normally open or conditioned for the transmission of sound, a sound impressed upon the microphone Ma would be reproduced by the associated loud speaker l5b at a comparatively high level of intensity, so that the reproduced and amplified sound would be impressed upon the microphone l lb and fed back along the channel 22 so as to be amplified by the loud speaker l5a in the room in which the sound originated. Thus objectionnation with each other as will hereinafter be pointed out.

In the form disclosed in Figs. 1' and 4 of the drawings the objectionable feed-back is prevented through electronic means which is automatically impressed upon either one of the transmission channels 2! or 22 to reduce the transmission effectiveness of the other of the paths Since each of the paths 2! and 22 normally includes amplifier means, such reduction in the transmission eifectivencss of the return path is attained through reduction in the amplification factor of actuated and controlled by sound or voice energy the amplifyingmeans of the return transmission path.

The transmission channels generally In the form of a lifying means 20 which is shown in 3n 4 of the drawings. each channel incluc I t o es of amplification, and the channels enerally similar in structure and operation, cl hence purposes of disclosure, only the of the channel 25 will be described in der vrein. The microphone i ls has a circuit connected thereto including a power source such as a 1.5 dry cell and also including the primary 24' of an input coupling transformer 24 whereby the microphone is connected to the amplifying means. The secondary 24" of the transformer 24 is shunted by an input control potentiometer 25, preferably of about 250,000 ohms, the adjustable contact 25 of which is connected by a lead 26 to the input grid 29 of a first amplifier tube 28. The tube 28 may be of the type and construction identified commercially as the type 6J7. The other or zero end of the potentiometer 25 is grounded at 29' by a lead 30, and from the lead 30 a wire 3| extends to the cathode resistor 32 which is in turn connected to the cathode 33 of the tube 28. The resistor 32 may be in the order of 600 ohms, and is shunted by a by-pass condenser 34 of 5. microfarads.

The plate-cathode circuit of the tube 28 is formed by a lead 35 from the plate 36 of the tube, a plate resistor 31 of about 100,000 ohms, a condenser 38 of about 16 microfarads, preferably electrolytic in character, and a lead 39, all connected in series to the juncture of the wire 31 and the resistor 32. The normal gain of the tube 28 is determined to some extent by the positive bias on a control grid 29" of the tube 28, this normal bias being ,applied by direct connection of the grid 29" ."to the cathode 33 of the tube 20. From the juncture of lead 35 and resistor 31, a lead t0, a coupling condenser 4| of .05 microfarad, and a lead 62, all in series, are connected to the control grid 43 of a second amplifier tube 3d. The tube 44 is preferably the tube known commercially as type 25L6. From the lead .02 a grid resistor 55 of 500,000 ohms is connected in series with a wire 46 to the lower end of a cathode resistor 41 of 300 ohms, this lower end of resistor being grounded at 29', while the upper end is connected to cathode 5B of the tube M. The cathode resistor til is shunted by an electrolytic condenser 01 of 10 microfarads.

The other grid 50 of the tube 54) is connected to one end of the coil 55 of an output transformer 52 by means of a wire 53, while the plate 54 of the tube 4 3 is connected by a wire 55 00 another point along the coil 5|. The speaker 150 is coupled to the coil 5| by a secondary 56, opposite ends of which areconnected to the terminals of the speaker. Preferably both the microphone circuit 23 and the secondary 56 of the speaker coupling are grounded to the frames of their respectiye cabinets as at 51. The ground 5?, and all grounds shown in dotted lines, are also connected to earth so as to constitute true ground connections. The other grounds such as wt, 29', 85a and mm, indicated by solid lines, are true grounds so far as audio frequency is concerned, but are not connected to earth except through condensers. This arrangement reduces danger due to shocks, andassists in reducing hum.

As hereinbefore pointed out, the other channel 22 is of the same construction and arrangement as the channel 2!, which has been described in detail, and for purposes of simplicity, corresponding reference numerals are applied to such corresponding elements of channel 22. as require designation or discussion, with such reference numerals prefixed in each instance by the numeral Thus the first amplifier tube of the channel 22, which corresponds with the tube 28. is designated 2-43; the second tube is designated 2*; while many of the corresponding elements of the channel 22 are not specifically numbered by reason of the fact that they do not a potentiometer I2 of 100,000 ohms to ground at require specific consideration in succeeding portions of this disclosure.

In the illustration of Fig. 4, the heater circuits have for purposes of simplicity, been eliminated.

The power supply As a power supply for the system of Fig. 4 I have illustrated a conventional A. C.-D. C. power supply unit I80 adapted for connection by an attachment plug I8I to any 110 volt line, and having terminals I62 and I83 to which the various tubes are connected. It will be noted that one point of negative potential in the unit I80 is grounded atI84 so as to provide a source for the several grounded cathodes of the system.-

The sound governed control means To attain the desired reduction in the ampliflcation factor of the second or return channel, the present invention utilizes the energy of the sound impressed on the first or desired channel, this control energy being applied through suitable intermediate means to the third control grid volving undue variation in gain or volume during the actual conversation over the two chan nels.

Thus a voice induced control potential or cur-- rent is taken bya wire 6I from the upwardly extended end 62 of the primary coil of the speaker coupling, this connection being in the nature of an auto transformer which impresses a relatively high potential on the wire 6I-. From the wire 6| the control current, which is pulsating direct current, is applied to branch leads 64 and 65 to a pair of similar control tubes 66 and 2-66,

I each branch lead 64 and 65 having a condenser 61 therein, of about .005 microfarad; which act to allow only the alternating components of the control current to pass to the control tubes 66 and 2-66.

Considered generally, the two tubes 66 and.

266 are energized by the alternating components of the control current from the wire 6I of the channel 2| so as to apply a positive potential to the grid'60 of the channel 2|, thereby to increase the normal gain of the channel 2|, and

also to apply a negative potential to the grid I 2-60 of the, channel 22, thereby to reduce the normal gain in the channel; it being undertstood,

of course, that this action is reversed in the event that the voice energy is'initially applied to the I channel 22.

To attain this desired functioning for both P channels, each control tube 66 and 2-66' is preferably in the form of a duo-diode (herein disclosed as a 6H6 tube), and the mode of connecting these tubes will be desrdbed in detail as applied to the channel 2|. Thus the tube 66 has one plate 69 connectedto wire 65 which carries the control current from channel 2i, while. its other plate I0 is connected to the wire 2- 6I which carries the control current from the 'channel 22. One cathode H of the tube as (the oath-. ode opposite the plate 10) is connected through 84a. The potentiometer I2 is shunted by a bypass condenser I2 of 10 microfarads, so that concerned, is at ground potential. The other cathode II of the tube 66 is similarly at ground potential as to A. C. currents, in view of the circuits which will hereinafter be described. Since one end of the potentiometer I2 is grounded it will be seen that the adjustment of the potentiometer will produce a predetermined adjustable positive potential which is applied through wires 8I and 84, resistor 83, wire 85, resistor I6, choke I1 and wires I8 and I9 to the grid 60 so as to determine the gain of the tube 28 during the cathode II, so far as alternating current is periods of inactivity or non-use of the channel 2|. As to the tube66, it will be evident that both diodes are inactive until the plates of the tube are rendered more positive than the oathodes II or II by the application of alternating currents to the plates thereof, and that either of these diodes may be rendered conductive independently of the other.

The plate 36 of tube 26 has a constant predetermined positive potential applied thereto through the plate resistor 31, the remote or lower end of which is connected through a, filter choke 30' of 30 henrys, and wires I90, I9I, I92, and I93 in series to the terminal I83 of the power supply. .From the lower end of the plate resistor 31 a filter condenser 38 of 16 microfarads is connected by means including wires 89 and 3| to ground at 29'. From the juncture of wires I90 and I9I a wire I94 extends to. the end of the potentiometer I2. Similarly a wire I 96 extends from the juncture of wires I9I and I92 to the lower end of the coil 5|.

Thus when the plate 69 is rendered more positive than the cathode II by the application of alternating current to plate 69, negative electrons gather on, the plate 69, and this flow is utilized to produce a relatively constant potential to be employed in increasing the gain in the tube 28. Thus,a resistor IS-oi 100,000 ohms is connected to wire 65, a resistor I6 of 250,000 ohms is connected to the other end of resistor I5, and, .the other end of resistor I6 is connected by a wire II" to the cathode II' of the tube 66. Around the resistor I6 a condenser I6 of-;25 microfarad is shunted.- From the lower end of the resistor I6, that is the end which is connected to cathode II, an audio choke II of 30 henrys is connected to a wire I8, which wire is connected to a wire I9 which extendsto the grid 60 of the tube 28. A condenser of .05 microfarad is connected between the juncture of resistors I5 and I6 and the juncture of wires I8 and I9. With this structure a flow of electrons toplate 69 establishes a relatively steady positive potential at the choke II which acts to increase the gain inthe tube 28.

This positive potential varies, of course, with the intensity of the voiceinduced'control current in the wire 6|, and ac- ,1 tually tends to reach an exceedingly high level which would produce an undue and. undesired objectionablylarge increase in the gainof; the tube 28, and 'as will hereinafter. be pointed out,

the present invention provides ionlimitingthe amount of gain which can be induced in. the channel by this variable positive potentiali" Considering the opposite channel as belngthel source of the voice-induced' control current, as'

when a sound is impressed on the channel '22,

v the control current corresponding to that above discussed, would be on the wire 2 -64, 'and the control potential impressed on the grid of the channel ZI would be negative rather than positive, and the gain in the channel 2i would be reduced. To accomplish this result, the platecathode circuitv of the plate I0 and cathode II has a resistance-capacitance filter system generally similar to the other diode of the tube 66. Thus a resistor 82 of 100,000 ohms has one end connected to wire 2-454, a second resistance 83 of 250,000 ohms being connected in series with the other end of resistor 82 to a wire 00 which is connected to the cathode II. A capacitance of 10 microfarads is connected between the wire 80 and ground at Ma; while a capacitance 82 of .1 microfarad is shunted around the resistor 83. From the juncture of resistors 02 and 83 a conductor strap extends to the juncture of resistors I5 and I6.

Thus when the diode 'III0 is rendered conductive by a voice-induced alternating control current in wire 260, the resistance capacitance filter system associated with plate 10 establishes a relatively. constant negative potential at the juncture of strap 85, with the filter system of the plate 10, and this negative potential is applied through the strap 85 and the other (and now inactive) filter system so that a negative potential is applied through the choke TI and wires I8 and 19 to the .grid 60. i

The limiting means for the sound governed control mary channel, and for limiting the voice-induced 7 reduction in gain which is produced in the other or return channel. This end is attained through a control tube I00 which acts to limit the increase in gain in either of the channels, and through a control tube IOI which acts to limit the reduction in gain in either of the channels. These tubes I00 and IOI may be of the 6H6 type as herein shown.

In the circuit herein shown, the tube I00 has both of its cathodes I02 connected to ground at I02a through a potentiometer I03 of 100,000.

ohms which is shunted by a by-pass condenser I04 of 10 microfarads. The otherend of the potentiometer I03 is connected by a wire I98 to the juncture of wires I92 and I93. Thusjthe cathodes I02 are connected to the power unit I00, so that an adjustable constant positive potential may be placed on the cathodes. The plate I05 of "the tube I00 is connected by wires I 06 and I06 to the juncture of the choke I1 and wire I8, so that theplate is charged with the same potential as the grid 60. Whenthis potential is positive and exceeds the threshold value established by the adjustment of the positive p0- aaoaieo threshold adjustment of the limiting tube I00.

A similar limiting action of the tube I00 on the voice-induced increase in gain in the tube 2-28 is attained by connecting the second plate I05 of the tube I00 to the grid 260 by wires I01 and I00.

The other control or limit tube IOI serves to limit the voice-induced decrease in gain which is introduced into either of the channels, and to this end the plates llfl and 0' of the tube I0! are connected in parallel by wires III and III to a constant source of negative potential such as the terminal I82 in the power unit I80. This connection establishes an operational threshold for the tube IN, and if desired, suitable means may be introduced in the lead III for variably adjusting the negative potential on the plates H0 and H0. One cathode II! of the tube IN is connected by a wire I I3 to wire I00 which leads to the grid 2-60 of the tube 228, while the other cathode H2 is connected by a wire M0 to wire I06 which leads to the grid 60 of the tube 28.

Thus when the voice induced negative charge on the grid 2-00 exceeds the threshold-establishing negative potential on plate I I0, the diode II 0---I I2 is rendered conductive; and the resulting current fiow in effect reduces the negative charge on grid 2-60 until the diode IIO-I I2 is again nonconductive. This action is for all practical purposes instantaneous, so that the gain in the tube 228 is never perceptibly reduced beyond the desired minimum.

The other diode IIO'I I2 of the tube IOI acts similarly to limit the voice-induced reduction in gain in the channel 28.

Operation of Figure 4 Summarizing the operation of the sound system of Fig. 4, the inter-related action of the various tubes and circuits will now be described for each direction of transmission. It will be understood, of course, that the system is ordinarily adjusted as to sensitivity, as by the potentiometer. I2, so that the various control elements will be insensitive or unresponsive to ordinary room noises.

When a user speaks into the microphone Ma of channel 2i, the pulsating voice-induced currents in the coupling 24 are impressed on grid 29 of the tube 28, and are amplified at the normal ratio in the plate circuit of the tube. The amplified potential on plate 36 of tube 28 is impressed on the control grid 43 of the second amplifier tube 44. The plate circuit of tube 04 carries the amplified voice-induced energy and energizes the coupling primary SI which. through its auto-transformer action supplies the requisite pulsating control current of high voltage in the wire 6|. For practical purposes this creation of the desired high voltage control current is instantaneous, since it takes place in a time interval so short as to be imperceptible to the human ear. This result is attained by making the time constant of the resistance capacity filter systems very small.

The alternating current components of pulsating control currentserve to provide the primary governing force for the desiredincrease in gain in the primary channel 2| as well as for the desired decrease or reduction in gain in the secondary or return channel 22. In attaining this purpose, the alternating components of the control current. from the wire 6| acts upon the diode 69'I I of the tube 06 to provide in its associated filter system the requisite positive potential upon the grid 60 of the channel 2|, while the current from the wire 6| acts through the diode 2-69- 2-II of the tube 2-66 and the associated filter system to provide the requisite negative potential upon the grid 2-60 of the tube 2-28.

When the increase in gain thus induced in the tube 28 tends to exceed the predetermined maximum determined by the threshold adjustment of the tube I00, the positive potential of the grid 60 acts on the diode I05-I02 of the tube I to render said diode conductive, thereby connecting the grid 60 to a source of less positive voltage which, in efiect, offsets the undesired excess of positive voltage which has been applied to the grid 60. itive charge on the grid 60 is limited by the threshold adjustment of the tube I00.

Similarly the negative or gain reducing charge on the grid 2-60 of the return channel 22 is limited by the diode being rendered conductive when the negative potential on the grid 2-60 exceeds the tube IDI.

When a user speaks into the microphone I412.

of channel 22, the voice induced currents in the coupling 2-24 are impressed on grid 2-29 of the tube 2-28, and are amplified at the normal ratio in the plate circuit of the tube. The amplified potential on plate of tube 2-28 is impressed on the control grid 2-43 of the second amplifier tube 2-44. The plate circuit of tube 2-44 carries the-amplified voice induced. energy and energizes the coupling primary 2-5I which through its auto-transformer action supplies the requisite pulsating control current of high voltage in the wire 2-6I.

This pulsating control current serves to provide the primary governing force for the desired increase in gain in the primary channel 22 and for the desired decrease in gain in the secondary or return channel 2|. In attaining this purpose, the control current from the wire 2-6I acts upon the diode 2-10-2-II' of the tube 2-66 to provide in its associated filter system the requisite positive potential upon the grid 2-60 of the channel 22, while the current from the wire 2-6I acts through the diode 10-II of the tube 66 and the associated filter system to provide the requisite negative potential upon the grid 60 of the tube 28.

When the increase in gain thus induced in the tube 2-28 tends to exceed a predetermined maximum determined by the threshold adjustment, of the tube I00, the positive potential of the grid 2-60 acts on the diode I05'-I02 of the tube I00 to render said diode conductive, thereby connecting the grid 2-60 to a source of less positive voltage which offsets the undesired excess of positive voltage which has been applied to the grid 2-60. The maximum positive charge on the grid 2-60 is thereby limited by the threshold adjustment of thetube I00.

Similarly the negative or gain reducing charge on the grid 60 of the return channel 2I is limited by the diode IIO-I I2 of the tube IOI, this diode being rendered conductive when the negative potential on the grid 60? exceeds or becomes more negative than the threshold-establishing negative potential on the plate 0' of the tube IOI.

Theacoustic system In Figs. 2, 2a and 2b of the drawings I have illustrated another means for reducing the eifective feed back of sound along the return path of As a practical matter the maximum pos-' a two-way automatic loud speaking communication system. This means of attaining the desired end may be termed an acoustic" system, since it is based upon the discovery that the predominant or natural frequency of the reproduced sound of the loud-speaker may be impressed upon the return channel in such a manner as to berendered ineffective as sound reproducing energy in such return channel. This end is attained by impressing such sound upon the microphone means of the return channel in two components which are substantially 180 out of phase with each other. One-means for attaining this outoi -phase input of the predominating frequency is shown in Fig. 2, wherein two microphones I400. are 'mounted at a distance apart which is equal to one-half the wave length of the natural or predominating frequency of the system. These two microphones I40a are arranged in the plane of the loudspeaker I50a, so that sound of the predominating frequcncy will reach the two microphones I40a in a phase displacement of 180.

' two microphones I40a may be. cancelled out by impressing them upon the same channel. In Fig.

2 this operation is illustrated by diagrammatically "showing the. sounds of the predominating fre'-- quency as circles of alternate'compressions and rarifactions'of the air, the rarifactions being represented by dotted arcs,- and the compressions "being represented by dash line arcs. Thus the microphones I40a are connected in series between a wire I5I and a wire I52, the wire I52 having a source of direct current such as a battery I53 therein. The wires I5I and I52 may be considered as a channel 2| which includes an amplifier I55 of conventional construction, the wires i5I and I52 being connected to the loudspeaker I50b of the'other station or set. The other channel 22' has two microphones I40b, mounted in the same manner as the microphones I40a. and connected in series between the wire I52 and another wire I54. The wires I52 and I54 therefore form the channel 22', which channel also includes the amplifier I55.

In attaining the desired out-of-phase application of the reproduced sound to the two microphones, I prefer to place the two microphones in a plane which includes the adjacent speaker, as shown in Fig. 2.

A similar cancelling out of the predominating frequency in the return path may be attained by the use of two loud speakers and a single microphone in each of the two sets as shown in Fig. 2a, the two speakers I of each set being spaced apart and related to the microphone III, in the same manner as disclosed in Fig. 2; in other words, the two speakers I10 are spaced apart a distance equal to one-half wave length'of the predominating frequency of the system, and are located in the same plane as the microphone I'II.

Another mode of attaining this cancelling-out action is shown in Fig. 21) wherein a suitable plane sounding board I13 is positioned vso that it reflects the sound to the single microphone I14 in the desired 180 phase relation to the sound reaching the microphone I14 by a direct path.

With the structure of Fig. 2, it will be apn The combined acoustic and electronic system In Fig. 3 of the drawings I have illustrated an embodiment of the invention wherein the novel amplifying and control unit of Figs. 1 and 4 is combined with the acoustic system of Fig. 2. Thus each set l6 of the Fig. '3 embodies two microphones I40 connected in parallel and spaced and arranged in the same manner as in Fig. 2. The two associated microphones I40 of each set I6 are operatively associated with their communication channel in substantially the same manner as has been described with relation to Fig. 4. Similarly, the system of Fig. 4 may be combined with the acoustic arrangements of Figs. 2a and 2b.

Thus an automatic reduction in gain in the a return channel is attained with the same general eifect as in the Fig. 4 embodiment, and in addition, the out-of-phase application of the sound .of predominating frequency to the return channel further reduces any tendency to produce oscillation in the system. Hence the combined system of Fig. 3 is highly flexible and can .be adapted to a wide variety of installation requirements.

Summary "nel enables the user to so adjust the system as to be-insensitive to normal room noises; and yet the user may speak into one of the transmitters at close'range' and instantly establish an increased gain in-the system which will transmit the desired message at a readily intelligible level.

Since the normal gain in the channels may be adjusted to a point sufliciently low to render the beadjusted to suit installation conditions, it will system insensitive to room noises, and the automatically attained operational level of gain may be evidenttliat the system of the present inven- -Z 'tionis extremely flexible and adaptable. 50

adaptability is founded in a large measure upon the reduction in gain which the present invention introduces in the return channel as .an incident to the transmission of sound in the first Drprimary channel. Since this reduction in ,gainmay be made relatively great so as to re- 'dl10 the gain to a relatively low level, which may insome instances practically block the return'path, it will be evident that the adjustment of the operational level of the primary channelis unhampered and unrestricted by the heretofore important possibility of feed-back and oscillation.

Furthermore, the limiting means which 'the present invention provides in association with the sound-induced control renders the system highly sensitive'and practically instantaneous "in response, since it enables the user to make the voice-induced control sensitive to sound only slightly above the normal room sound level, and yet with this gijeat sensitivity the louder sounds of desired conversation are reproduced at a sub- *stantiallynormal level at the reproducer.

' With the acoustic system herein disclosed a satisfactory suppression of feed-back and oscillation is. obtained in many instances without undue cost or modification of existing equipment; and when this acoustic treatment is combined with the electronic system of the present invention, the resulting structure is one which will meet even a wider variation of requirements in installation or use.

I claim as my invention:

1. A communication system comprising means providing two normally active telephonic communication channels each having an input trans mitter and loud speaking reproducer, the transmitter of each channel being located adjacent to the loud speaking reproducer of the other channel, and means operable by a relatively low intensity of sound energy impressed on one of said channels to reduce the transmission eificiency of the other of the channels and simultaneously to increase the transmission efliciency of said one of the channels. I

2. A communication system comprising means providing two normally active telephonic communication channels each having an input transmitter and loud speaking reproducer, the transmitter of each channel being located adjacent to the loud speaking reproducer of the other channel, each of said channels including variable amplifying means of the electronic type,

means operable by relatively low level sound energy in one of said channels to produce a control potential variable between zero potential and a predetermined maximum while the sound impressed on said one channel varies within relatively low range of intensity and maintained at said predetermined maximum while said impressed sound is above said range, andmeans for applying said control potential to the amplifying means of one of said channels to vary the amplification factor thereof.

3. A communication system comprising means providing two normally active telephonic communication channels each having an input transmitter and loud speaking reproducer, the transmitter of each channel being located adjacent to the loud speaking reproducer of the other channel, each of said channels including variable amplifying means of the electronic type, means operable by relatively low level sound energy in one of said channels to produce a control potential variable between zero potential and a predetermined maximum while the sound impressed on said one channel varies within relatively low range of intensity and maintained at said prede termined maximum while said impressed sound is above said range, and means for applying said control potential to the amplifying means of the other one of said channels to vary the amplification factor in said other channel.

4. In a communication system having means providing two normally, active telephonic communication paths each having an input transmitter and a loud speaking reproducer, the transmitter of each channel being located adjacent to the loud speaking reproducer of the other channel, and each channel including grid controlled variable gain amplifying means, a conductor connected to a first one of said channels at apoint carrying pulsating direct current when voice energy is being transmitted along said first channel, a thermionic valvei operatively associated with said conductor by means including a capacitor to render said valve conductive when the alternating currents of a pulsating direct current on said conductor are applied through said capacitor to said valve, a resistance- Icapacity filter operatively connected to the plate-cathode cirdetermined value.

5. A two-way communication system of the character described comprising a pair of communication channels each including amplifying means adapted normally to amplify at a relatively low ratio and including a control grid for varying said ratio,.means,operatively connected to one of said channels and controlled by voice induced currents in said one channel to develop a relatively high control potential, and means for applying said control potential to the grid of one of said amplifying means to vary said normal amplification ratio, said last mentioned means being operable to limit said potential to a predetermined value.

6. In a two-way loud speaking communication system having two channels of communication and an individual amplifier for each of said channels, the combination of control means for said amplifiers operable by sound energy in either channel to introduce a predetermined reduction in the amplification factor of said amplifier of the other or return channel, andto introduce a predetermined increase in the amplification of the channel in which said sound energy was first impressed.

'7. In an electrical control circuit, the combination of a diode, an energizing lead for applying alternating current to render said diode conductive, a resistance capacity filter system in circuit with said diode, a useful voltage tap on said filter system for taking oil a potential of a given p'olarity, and a control diode having one of its elements connected to said tap so as to be charged with the same polarity potential as said tap and having its other element charged with an opposite polarity potential of a predetermined threshold-establishing value. I I

8. In an electrical control system, a vacuum tube having a pair of diodes, individual energizing circuits for rendering said diodes conductive selectively, a resistance capacity filter system associated with each of said diodes, an output circuit from one of said filter systems, and means inter-connecting said filter systems whereby said output circuit is charged with .a negative potential when one of said diodes is conductive and is charged with a positive potential when the other of said diodes is conductive.

9. An electrical control system as defined in claim 8 wherein a first governing diode has its plate connected to said output circuit'and its cathode charged to a. predetermined positive threshold establishing potential, and a second overnin diode'has its cathode connected to said output circuit and its plate charged to a predetermined negative threshold-establishing potential 1f In an amplifying transmission system, the

combination of intensity controlled expander means operable to increase the gain in the system in response to sound energy in the system,'

and means operable to limit such expansion to a predetermined level.

11. In a transmission system having grid-controlled electronic amplifying means, the combination of control means operable to produce a control potential in response to'and varying in proportion to'sound energy impressed on said amplifying means, a grid to which said control potential is applied, and means operable to limit the effective intensity of said control potential on said grid to a predetermined level.

12. In a two-way loudspeaking'communication system of the automatic or constantly active type and providing two communication channels each including transmitter means and loud speaker reproducing means, both of said channels terminating in remotely located stations each of which includes the transmitting means of one of said channels and the reproducing means of the other of said channels, one of said means of each station comprising two individual transducing devices connected in series to their channel and spaced apart a distance equal to one-half wave length of the natural period of the system.

13. In a two-way loud speaking communication system of the automatic or constantlyactive type and providing two communication channels each including transmitter means and loud speaker reproducing means; both of said channels terminating in remotely located stations each of which includes the transmitting means of one ofsaid channels and the reproducing means of the other of said channels, the transmitting means of each station comprising two individual transmitters connected in series and spaced apart a distance equal to one-half wave length of the natstation comprising two individual reproducersconnected in series and spaced apart a distance equal to one-half wave length of the naturalpe 'riod of the system.

15. In a two-way loud speaking communication system of the automatic or constantly active type and providing two communlcatlonchannels each including transmitter means and loud speaker reproducing means, both of said channels terminating in remotely located stations each of which ineludes the transmitting meansof one of said channels and the reproducing means or the other I of said channels, thetransmitter means of each of said stations including a single microphone which is subjected to the sound reproduced by the reproducer of its station and travelling from the reproducer to the mlcrophqne along a direct path, and planar sound reflecting means operable Y to reflect such reproduced sound tosaid' microphone in phase displacement with. relation to the same sound reaching said microphone by themostdirectpathxf 7 I HAROLD J. McCREARY. 

